This invention generally relates to directional solidification furnaces and, more specifically, to an apparatus and method for directing an inert gas flow into the furnace.
Directional solidification furnaces are often used in the production of multi-crystalline silicon ingots. Raw silicon is first loaded into a quartz crucible. The silicon can take the form of solid chunks, recycled polysilicon, silicon dust, or a combination of them. The crucible is typically constructed of quartz, or another suitable material that can withstand high temperatures while remaining essentially inert. The crucible is typically a five-sided box, with the top of the box being open to the atmosphere within the furnace. The quartz crucible is supported by graphite support walls that add structural rigidity to the crucible.
After the crucible has been charged with silicon, the area surrounding the crucible is sealed from the outside ambient environment. To aid in the separation of the crucible from the outside environment, the crucible is placed within a containment vessel that forms part of the furnace. The pressure within the containment vessel is then reduced. The content of the atmosphere within the containment vessel can also be monitored and controlled.
The crucible and the charge are then heated to a temperature sufficient to melt the silicon. After the charge has completely melted it is cooled at a controlled rate to achieve a directional solidification structure. The controlled rate of cooling is established by any combination of reducing the amount and location of heat applied by the radiant heaters, the movement of or the opening of a heat vent in insulation surrounding the crucible, or the circulation of a cooling medium through a cooling plate. Any of these methods transfer heat away from the surface of the crucible. If the rate of cooling of the bottom of the crucible is greater than that of the sides of the crucible, then a relatively flat, horizontal solidification isotherm with predominately axial thermal gradients is generated. The ingot thereby solidifies in the region closest to the cooler side of the crucible and proceeds in a direction away from that side of the crucible. The last portion of the melt to solidify is generally at the top of the ingot.
A significant concern in the production of multi-crystalline silicon ingots in directional solidification furnaces is the contamination of the ingot with impurities. An entry point of contamination is often at the melt surface. Gaseous or solid carbon or other contaminants present in the containment vessel enter at the melt surface and are at least partially absorbed by the melt, and are subsequently incorporated into the ingot upon solidification sometimes as a precipitated compound. Some of the sources of carbon contamination are carbon monoxide gas formed when the crucible and walls are heated, as well as oxygen-containing compounds contacting heated insulation and graphite or the deterioration of friable insulation surrounding the walls and the interior of the furnace enclosure resulting in particulate carbon contamination. The carbon monoxide gas is formed in the following reaction: SiO(g)+C(s)SiC(s)+CO(g) and O2(g)+2*C(s)2*CO(g) where the source of the gaseous SiO can be evaporation from the free silicon melt surface or decomposition of the crucible by the reaction 2*SiO2(g)2*SiO(g)+O2(g), and the oxygen may originate from air remaining in the furnace or air leaks into the furnace. Decomposition of the quartz crucible by the adjacent crucible support is also an important source of carbon containing species. This occurs by a reaction such as SiO2+2*C(s)SiC(s)+CO(g) The carbon monoxide or carbon dioxide gas (created from any source) reacts with the melt surface as represented by the following reactions: Si(l)+CO(g)→SiO(g)+C or Si(l)+CO2(g)2*SiO(g)+C.
Carbon precipitates in silicon ingots cause undesirable electrical shunts in the products that are eventually fabricated from the ingots, such as solar cells. Carbon also contaminates the recycle stream of silicon, as unused or unsatisfactory ingots are often recycled to form new ingots. Consequently, reducing the carbon contamination of the melt decreases the carbon contamination levels of the recycle stream.
Attempts have been made to introduce an inert gas flow in the furnace, but they have not been completely satisfactory due to an ineffective flow path. Accordingly, an efficient and effective apparatus and method are needed to introduce an inert gas flow to reduce contamination levels in the ingot.